Ohmic contact electrode to semiconducting ceramics and a method for making the same

ABSTRACT

AN ELECTRODE COMPOSITION, FOR OHMIC CONTACT TO AN OXIDE SEMICONDUCTING CERAMIC, CONSISTING OF A NICKELPHOPHORUS ALLOY, THE ALLOY CONTAINING 85-99.5% BY WEIGHT OF NICKEL AND 0.5-15% BY WEIGHT OF PHOSPHORUS, AND A METHOD FOR MAKING SAME BY ELECTROLESS PLATING.

TSUNEHARU NlTTA ET AL 3,555,376 OHMIC CONTACT ELECTRODE TO SEMICONDUCTING CERAMICS ;AND A METHOD FOR MAKING THE SAME Filed Dec. 5. 1966 ATTORNEYS United States Patent O i A'BSTRACT-*OF 'THE'DISCLOSURE :An electrode' 'co r'nposition; for ohmic contact to an ox ide' semiconductin'gf^ceramic, `consisting of a nickelphosphor isj alloy', the alloy jcontaining 85-99.5% by weight 'of nickel and (pS- 15% byweight of phosphorus, and' a 'method for making same by electroless plating.

-This invention relates `'to an ohmic contact electrode to be applied to semicon'ducting ceramics and more particularly to a method for making the same by 'electroless plating. v I v v A low-resistance ohmic contact to ferrite and semiconducting titanate ceramics has'been effected by rubbing indium wettedwith mercury or gallium on the surface. A drawback of the contact producedby this method is low` mechanical strength, low stability with temperature and difi'iculty in. soldering lead'wires thereon.-Recently, D. R. Turnerand'H. A. Sauer -have disclosed that a nickel contact obtanedby electroless plating produces a low-resistance ohmic contact to semiconducting ceramics such as barium titanate' (Journal-'of the Electrochemical Soci-ety, vol.' l07No. 3, p. 250 .1960). According 'to their report, the nickel 'contact formed on the ceramics by the' electroless plating is nonohmic as deposited and is required to be heated at about 170 C. for obtaining a satisfactoryohmic contact.

Therefore, it is a principal 'object of this invention to provide a method formaking an ohmic contactto a sem-iconducting ceramic by employing electroless plating in a more simple manufacturing step.

'It is a further object of the invention to provide an ohmic contact electrode composition characterized by high mechanical strength and high stability with temperature and time.

It is another object of the invention to provide an electrical circuit element comprising a semiconducting- FIG. 1 is a cross-sectional View of a ceramic body containing an electroless deposited alloy in accordance with this invention.

FIG. 2 is a cross-sectional View of a thermistor djevicejin accordance with this invention.

According to the invention, it. has been discovered that nickel-phosphorus alloy is strongly attachedto oxide semiconducti ng ceramicsby a methodwhich comprises depositing nickel-phosphorus ;a11oybyche mical reluction, ie. by an electroless process, onappropriatetsurfaces of the ceramics and that nickelrphosphorus alloy produces a low-resistance ohmic contact without any additional procedures. Oxide semiconducting ceramics pro- Patented Jan. 12, 1971 vided with said nickel-phosphorus alloy contact can be used as electrical crcuit elements such 'as a thermistor, a resistor and an electric heate' of high reliability in accordance with the invention.

The process according to the invention is based on the controlled autocatalytc reduction of nickel cations by means of hypophosphite anions in the presence of water. The significant gross equations can be written:

Ni++ [H2P 02]- HzO N ickel Hypophosphite Water Cation .Anion Ni 2H+ H[HP 031" Metallic Hydrogen Acid Ortho- Niekel Cations plosphite Anions Concurrently, more hypophosphite anions are oxidized to orthophosphite anions with evoluton 'of gaseous molecular) hydrogen through the catalytc action of the metallc nickel formed in reaction (I) Catal.(Ni) [H2P 02]` HzO H[HP OaH-Hz T [II] Furthermore, the hypophosphite anion is reduced by atomic hydrogen, yielding elemental phosphorus, water and hydroxyl ion according to the equation:

The elemental phosphorus is immediately bonded to the nickel, making the reaction irreversible. Equation (III) clearly shows that the deposition of phosphorus in the electroless solution is dependent on the pH value of the solution. Thus, nickel-phosphorus alloy can be obtained by the above-mentioned process.

Referring now to FIG. 1, the disc-shaped n-type semiconductive barium titanate ceramic 1 is provided with nickel-phosphorus alloy 2 by depositing nickel-phosphorus alloy by the aforedescribed electroless process.

An alloy containing a phosphorus content higher than 15% by weight is not desirable for use in a contact electrode because the high amount of phosphorus 'esults in an extensive oxidation at the free surface and prevents good electrical or mechanical connection to a soldered lead wire. Pure nickel or the alloy containing a phosphorus content lower than 0.5% by weight cannot be utilized for application of an ohmic contact electrode because a higher resistance barrer layer is forrned between the semiconductive oxide ceramic and the electrode. The following proportions of. phosphorus to nickel can be used:

The semiconductive barium titanate ceramic can be prepared by mxing barum carbonate andjtitanium oxide in an equimolecular ratio with or without ad dition of au small amount of eXcess of titanium oxide, pressing` into the form of a disc, and-sintering at about 1300 to-1400 atmosphere such as `nitrogen and' C. in a nonoxidizing argon.

Thewmixture may" be-doped with rare earth element: calcined at `8001to` oxide. and, whent desired,` may ,be 1.000? C. prior tosintering.- i 4,

The process of.-depositing nickel-phosphorus' alloy on" an oxide ceramic semiconductor is carried outrby thefol-` lowing method:- (1) in advance of thedepositing treat ment, the ceramic object, which is to have its surface&

coated, is cleaned by any ofithe acceptable andsconventiom al procedures, and (2) the area to be coated with nickelphosphorus alloy is activated by dipping for, about one TABLE 2 Grams NiCl 6HO 3.0 NaHzpog Hgo 1.0 Na C H O 1 H O Remainder Total 100 The soluble nickel chloride serves as the source of the nickel ions and the nickel depositon, and the hypophosphite serves as the reducing agent and the source of the phosphorus. Sodum citrate serves to hold nickel salts in solution. The pH value of the solution can be controlled, in a conventional manner, by addition of an aqueous solution of ammonium chloride, ammonium hydroxide or hydrogen chloride. The ammonium chloride and ammoni um hydroxide serve to maintain the solution alkylinity and also to hold the nickel salt in solution. The hydrogen chloride serves to maintain the solution acidity.,

Phosphorus-concentrations of deposited alloys increase as the pH value decreases from 10 to 2. Good results can be obtained by employing a solution of a pH value of 3 to 10. Table 3 shows compositions of deposited alloys determined by chemical analysis as a function of the pI-I value.

TAB LE 3 Compositions (pocentagcs by weight Nickel pH value of the solution Phosphorus HH 99 99 5 3 ws ootoooee The alloy contact according to the invention has such high resistance to oxidation that no appreciable oxidation is observed up to 600 C. in air though pure nickel contact according to the prior art oxidizes easily when heated above 300 C. in air. It has been discovered that said alloy contact is improved in bonding strength to the semiconducting ceramic and in mechanical hardness. The conventional nickel contact is required to -be heated at about 170 C. for achieving an ohmc contact to the ceramic. The novel alloy contact exhibits an ohmc contact to the ceramic as deposited. Therefore, heat treatment according to the invention has different purpose from that of the prior art.

It is preferable for obtaining a high bonding strength and a high mechanical hardness to heat said alloy contact applied to the ceramic below 600 C.'in air.

Referring to FIG. 2, the disk-shaped n-type semiconductive barium titanate ceramic 3 is provided` with electrodes 4 at both surfaces of the ceramic by depositing nickelphosphorus alloy and subsequently heating the contacts below 600 C. in air, Leadwires are soldered to the electrodes .4 with a lead-tin solder 6. The ceramic 3 is shown as "a flat wafer, but it maybe a pellet or rod or of other" shape. j y I i The following examples of presently-preferred embodiments are gven by'way of ill'ust'aton'and should not be' con-trued as l'imitztive.-

EXAMPLES 1 "TO 8 Semiconductive barium titanate is made in per se conventional manner. Equimolecular mixtures of titanium oxide and barium carbonate are wet milled with 02% of silver oxide, pressed into a disc mm. in diameter and 3 mm. thick and sintered at 1300 C. for 2 hours in nitrogen atmosphere. The sintered body, in blue-black color, is 8 mm. in diameter and 2 mm. thick and has ohms of electrical resistance. Various contact electrodes as listed in Table 4 were attached on the semiconductive barium titanate ceramic by the following process.

The ceramics are cleaned in a boiling aqueous solution of 5% of sodium hydrocarbonate (NaHCO and the area to be coated with alloy is activated by dipping first for one minute in an aqueous solution of 1.0% of stannous chloride (SCl and subsequently for one minute in an aqueous solution of 0.0001% of alladium chlo-` ride (PdCl The objects to -be coated .are immersed in the electroless bath composed of an aqueous solution of 3.0 grams of nickel chloride, 1.0 gram of hypophosphite, 1.5 grams of sodium citrate, and ammonium chloride and ammonium hydroxide or hydrogen chloride fortadjusting the pH value per 100 grams of said 'solution: Ten minutes in the electroless solution (solution temperature -95 C.) deposit a layer of nickel-phosphorus alloy about one micron thick. Electrical resistivity is measured by a per se well know method, and is shown in Table 4. The samples containing nickel-phosphorus alloy consisting of 90 to 99% by weight of nickel and 1 to 10% by weight of phosphorus result in low-resistance ohmc contacts. Tests of the resulting electrical circuit element devices indicate that the resistance of the entire group averages closely to 20 ohms, which is essentially the resistance of'the semiconductive material itself. In addition, the electrodes so produced have an excellent bonding strength. Fifty samples are subjected to a test cycling from 70 C. to 200 C. 'by 2,000 cycles. After the test, forty-fivesamples show no change in the electrical resistance and only five samples show a slight increase in the electric resistance. This is indicative of the extremely high stability and the low ohmc resistance of the contacts applied to the devices in accordance with the present invention.

While the foregoing is specific to applying nickel-phosphorus alloy coatings to the particular semiconductor composition described herein, it should be understood that said coatings may be applied with equal success to oxde ceramic semiconductor bodes comprising other compositions such as ferrites, thermoelectric members, etc.

TABLE 4 Electriual resistance composition of alloy in at 25 C. at pl-I value of weight porcentages 1 volt r electroless (D.C 00 Sample No. solution Ni P (olnn-cn.)

1 2 83.2 16.8 23.0 2 3 85. 7 14. 3 19. 8 3 4 87. 2 12. 8 20. l 4 5 91. 8 8. 2 20. 3 5 (i 93. 2 6. 8 20. (l 8 04. 3 5. 7 20. 4 10 06. (i 3. 4 20. G 12 99. 7 0. 3 (i, 230

6 nckel-phosphorus alloy, said alloy being composed of 3,443,170 5/ 1969 Pulvari 317-234 90 to 99% by weight of nickel and 1 to 10% by Weight of 3,419,758 12/1968 Nitta et al. 317-230 phosphorus.

4. An ohmic contact electrode composition as claimed FOREIGN PATENTS in claim 3, wherein said oXide semiconducting ceramic 5 437 263 6 1938 Great Britain 31 234 comprises semiconductive barium titanate.

References cited JOHN W. HUCKERT, Prmary Examner UNITED STATES PATENTS M. EDLOW, Assistant Examiner 3,208,139 9/1965 Wijburg 29-492 10 2,922,73o 1/1960 Feldman 117-221 3,331,996 7/1967 Green 317-234 317 234; 31o .9,4, 9 5 

